Polyphosphoinositides, generation of second messengers, myosin light chain phosphorylation and contraction in rabbit iris sphincter smooth muscle

Abstract

There is general agreement now that elevation of intracellular calcium ion concentration [( Ca2+]i) mediates the physiological actions of a number of hormones, neurotransmitters and other pharmacological agents. In smooth muscle one way by which these agents elevate [Ca2+]i and induce contraction involves formation of inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DG) from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2). Our initial observation that the muscarinic-stimulated breakdown of PIP2 into IP3 and DG in the iris smooth muscle is a primary event that could couple activated receptors to contraction is now supported by the following: 1. In the iris sphincter contractions by carbachol (CCh) indicated close correlations, using different concentrations of CCh, atropine- and pirenzepine antagonists, time course, temperature and Ca2+, between the stimulated hydrolysis of PIP2, myosin light chain (MLC) phosphorylation and muscle contraction. Further, studies on the relationships between receptor occupancy of muscarinic acetylcholine receptors in this tissue, measured by [3H]QNB binding, and the CCh-stimulated PIP2 hydrolysis, MLC phosphorylation and contraction revealed that all of these responses are coupled to the M2 receptor subtype. 2. NaF, which activates G proteins by promoting the dissociation of their subunits, produced a concentration-dependent activation of phospholipase C, measured as IP3 accumulation, and contraction in the iris sphincter muscle. 3. In permeabilized smooth muscle fibers, IP3 has been shown to release Ca2+ from the SR and induce contraction. 4. In the iris sphincter, as well as in other smooth muscles, phorbol 12,13-dibutyrate, which mimics the action of DG, induced MLC phosphorylation and contraction in a dose- and time-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)